1. Field of the Invention
The present invention relates to a via structure of a printed circuit board (PCB). More specifically, the present invention relates to the via structure of a PCB for transmitting differential signals.
2. Description of the Related Art
It is known to use differential signaling to transmit information. Differential signaling uses two complementary signals that are sent on two paired transmission lines, e.g. contacts, wires, or traces. These paired transmission lines are referred to as differential pairs, and the complementary signals are referred to as differential signals. The differential signals are typically transmitted through a connector and a PCB. In the connector, the differential signals are transmitted through an array of contacts. The array of contacts is connected to an array of vias within the PCB. The arrangement of the array of vias is similar to the arrangement of the array of contacts. The PCB includes a break out region (BOR) in which the differential signals are routed to different portions of the PCB. Typically, multiple layers of the PCB are used in the BOR so that the differential signals can be routed on different layers of the PCB.
FIG. 16 shows a plan view of a known footprint of a PCB 110. The footprint shows the vias 101 arranged in an array with 50 mils×50 mils (1.27 mm×1.27 mm) (where mils is equal to one thousandths of an inch and mm is millimeters) pitch, where adjacent vias 101 are spaced 50 mils apart in both the top-to-bottom and left-to-right directions. The vias 101 are connected to corresponding contacts 102 of a connector (not shown in FIG. 17) with solder 103 as shown in FIG. 17. For simplicity, only a portion of the contacts 102 is shown FIG. 17. This specification uses the convention that for reference numbers that include reference numbers without letters and the same reference number with letters that the reference number without a letter, e.g. 102, refers to all corresponding elements, e.g. all contacts, while the reference numbers with letters, e.g. 102a, 102b, 102g, refer to specific elements, e.g. contacts 102a, 102b, 102g as shown in FIG. 17. The contacts 102 are arranged in a similar array as the vias 101. FIG. 17 only shows a portion (a four-by-four array) of the array of vias 101 and contacts 102 shown in FIG. 16.
FIGS. 17 and 22 show that the width of the channels available for routing the traces 105bo between the vias 101 in the BOR is limited to 50 mils minus the via plated through hole (PTH) size, which limits the possible trace routing options in the BOR.
In FIG. 17, contacts 102a, 102b are paired contacts that transmit complimentary signals, i.e. contacts 102a, 102b are a differential pair. Ground contacts 102g are arranged around contacts 102a to improve signal integrity of the differential signal transmitted through the contacts 102a, 102b by, for example, shielding the contacts 102a, 102b from adjacent differential pairs.
FIGS. 18 and 19 show another conventional via structure in which the contacts 102 are connected to pads 108 by solder 103. The contacts 102 are electrically connected to the vias 101 by traces 105. Each of the vias 101 includes an annular ring 104 that is connected to the corresponding trace 105.
The prior art via structures described above fail to include a single central axis of signal propagation through the transition from the connector to the PCB. As seen in, for example, FIG. 21, the central axis of signal propagation through the contacts 102 of the connector is different from the central axis of signal propagation through the vias 101. This difference in the central axes is determined by the traces 105. The central axis of signal propagation through the two contacts 102a, 102b of a differential pair is in the center between the two contacts 102a, 102b. Similarly, the central axis of signal propagation through the vias 101a, 101b of a differential pair is in the center between the two vias 101a, 101b. The central axes are offset from each other by the length and direction of the traces 105, which is typically 36 mils for a 50 mils pitch. Further, the prior art via structures lack angular symmetry with respect to the top ground plane layer and lack preferential coupling between the vias of the differential signals because the cross term coupling factors are evenly distributed. The failure to include a single central axis of signal propagation, the lack of angular symmetry, and the lack of preferential coupling negatively affect signal integrity.
In FIG. 18, because the antipad 107 (i.e., holes or portions where the ground plane 106 is not located) of the ground plane 106 encircles only the vias 101a, 101b and does not encompass the pads 108a, 108b and traces 105a, 105b when viewed in plan view, the capacitive coupling between the pads 108a, 108b, traces 105a, 105b, and the ground plane 106 is increased. Too much capacitive coupling can cause a drop in the time-domain reflectometer (TDR) impedance profile, which can cause the signal to be reflected back and not transmitted. If the size of the antipad 107 was increased to remove the ground plane 106 from underneath the pads 108, then the larger antipad 107 would increase crosstalk and affect the impedance profile. Also, as the signal speed increases, larger antipads 107 can cause the signal to change propagation modes around the antipads 107, which can further cause signal loss and reflections.
As shown in FIG. 20, each ground contact 102g is connected to a corresponding ground via 101g, which increases costs because each ground via 101g connected to a ground contact 102g must be drilled.